In the medical field, the ultrasound diagnosis apparatus is used for various diagnoses and treatments because it can investigate the internal structure, blood flow state, and the like of the subject non-invasively. The ultrasound diagnosis apparatus transmits ultrasound waves into the body from an ultrasound probe having a transducer (piezoelectric transducer) at its tip and brought into contact with the body surface of the subject. Then, the transducer of the ultrasound probe receives reflected waves caused by the acoustic impedance mismatch inside the subject. The ultrasound diagnosis apparatus generates an ultrasound image based on the received signal obtained in this way.
When the ultrasound diagnosis apparatus generates the ultrasound image, the reflected waves (received echo) received by the transducer are amplified by a preamplifier in a receiving circuit. However, unless appropriate gain control is performed on the preamplifier, the preamplifier saturates, and an appropriate ultrasound image is not to be generated and displayed. As described above, the ultrasound diagnosis apparatus generates the ultrasound image using the reflected waves received from the subject, and therefore, is required to transmit and receive ultrasound waves as accurate as possible.
Meanwhile, in recent years, a technique called harmonic imaging has been developed. The harmonic imaging uses a nonlinear component detected in a trace amount when a sound wave propagates with respect to the generation of an ultrasound image. An ultrasound wave has the property that it propagates in a high sound pressure part at a higher speed than in a low sound pressure part. Therefore, even if the transmitted ultrasound waves are sinusoidal waves composed of reference wave components, the distortion gradually occurs in the course of the propagation. As a result, the ultrasound waves include harmonic waves having nonlinear components.
Examples of the harmonic imaging using harmonic components include contrast harmonic imaging (CHI). In CHI, a subject is administered a contrast medium containing microbubbles for ultrasound waves to image harmonic components generated when the microbubbles resonate and collapse.
Therefore, in CHI, it is necessary to use microbubbles without destroying them. However, even in the case of ultrasound irradiation used in normal diagnosis, the microbubbles can be destroyed by the mechanical action of ultrasound waves. If the microbubbles are destroyed, the intensity of reflected signals is deteriorated. Therefore, in order not to destroy the microbubbles as much as possible, there is a demand for imaging by ultrasound transmission at a low sound pressure level. On the other hand, the imaging by ultrasound transmission at a low sound pressure level raises concerns about the reduction in S/N ratio.
In CHI, reference wave components and higher harmonic components are separated by filtering and waveform computation to extract harmonic components. Although there are various methods for extracting the harmonic components, in order to solve the problem in imaging by ultrasound transmission of low sound pressure, for example, a method called amplitude modulation is used. In the amplitude modulation method, ultrasound waves are transmitted three times such that the ratio of the relative sound pressures of them becomes 1:2:1. Then, the sum of two received signals having a transmission sound pressure 1 is subtracted from a received signal having a sound pressure 2 to remove the reference wave components, thereby extracting the harmonic components.
In this manner, in CHI, ultrasound waves are transmitted and received a plurality of times with different sound pressures to extract the harmonic components. In order to set the transmission sound pressures 1 and 2 to different values, voltages applied to transducers may be set to different values; however, it is difficult to set the applied voltages to different values with high accuracy. Therefore, all the transducers are operated for the transmission sound pressure 2, while half of the transducers are operated for the transmission sound pressure 1 to adjust the sound pressure. In the case of setting the transmission sound pressure 1, to match the transmission apertures with those of the transmission sound pressure 2, odd-numbered or even-numbered transducers (i.e., every other channels) of the transducer array are operated.